When plastics are injection molded, a gate is connected to a mold cavity to inject the plastic material into the cavity. This gate can assume various forms, such as side gate, pin gate, fan gate, disk gate, and film gate. Except for the pin gate, the article and the runner are taken out of the mold usually as a unit after they are connected together at the gate. The molded article is degated in a subsequent step to complete the article.
Conventional degating methods are disclosed, for example, in U.S. application Ser. No. 348,820, now U.S. Pat. No. 4,952,354 entitled "Degating Method" and Japanese Patent application No. 188730/1989. In these techniques, a member constituting a gate is made slidable relative to a mold cavity or a core on a plane that is inclined at about 90.degree. with respect to the axial direction in which the mold is opened. After the gate is completely sealed subsequent to the filling of the material or during dwelling or pressure holding, the member constituting the gate is vibrated or reciprocated with high frequency. The energy of the vibration softens the plastic around the gate. When the plastic softens sufficiently, the member is displaced to degate the plastic.
In the above-described degating method, the degating leaves no traces and, therefore, it is possible to fabricate aesthetically excellent products. This known method is effective for gates connected to narrow runners, but the energy of the vibration is not transmitted to wide gates such as film gates. For this reason, it has been impossible to finish the degated surface as desired, using wide gates. More specifically, in the conventional method, the sliding member constituting the gate of the mold is displaced on a plane inclined at 90.degree. with respect to the axial direction in which the mold is opened and closed. This method is effective for gates having small cut areas. However, such gates are very few in number. The conventional method cannot be applied to general molds having broad planar portions.